We began our work to discover genes required for expression or activity of fusion oncogenes by using the results of a genome-wide RNAi screen designed to identify genes needed for the activity of the fusion oncoprotein EWS-FLI1. This RNAi screen was developed in collaboration with researchers from CCR's Pediatric Oncology Branch and the trans-NIH RNAi screening facility at the National Center for Advancing Translational Sciences (NCATS). EWS-FLI1 is an oncogenic transcription factor that is generated as a consequence of chromosomal rearrangements involving chromosomes 11 and 22. This is the initiating genetic event observed in the majority of Ewing sarcomas (ES), cancers of the bone and soft tissue. The RNAi screen revealed the expression of the EWS-FLI1 fusion transcript is sensitive to the inhibition of the activity of specific splicing factors. We identified that the splicing factor HNRNPH1 is required for the expression of the EWS-FLI1 transcripts expressed in ES cells harboring translocations where the breakpoint in chromosome 22 occurs within a specific region of DNA. Depleting HNRNPH1 in ES cells where the breakpoint occurs within a specific region disrupts the expression and the activity of the EWS-FLI1 protein and reduces cell survival. Furthermore, we identified SF3B1, a major component of a protein complex required for splicing as a protein that ES cells are particularly dependent upon.